Transient compressor surge response for a turbocharged engine

ABSTRACT

A method for responding to an existing or incipient surge condition of a turbocharger coupled to an engine of a motor vehicle is provided. The method comprises receiving a signal responsive to an operating condition of the turbocharger and adjusting one or more operating parameters of the motor vehicle when a power of the signal, integrated over a pre-selected range of non-zero frequencies, exceeds a pre-selected threshold. Other embodiments provide related systems for responding to an existing or incipient surge condition of a turbocharger.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application61/056,620, filed on May 28, 2008, and entitled METHOD OF TRANSIENTCOMPRESSOR SURGE DETECTION FOR A TURBOCHARGED INTERNAL COMBUSTIONENGINE, the entirety of which is hereby incorporated herein by referencefor all purposes.

TECHNICAL FIELD

The present application relates to the field of internal combustionengines, and more particularly, to improving the reliability ofturbocharged internal combustion engines of motor vehicles.

BACKGROUND AND SUMMARY

A turbocharger may be used to increase the power output of an internalcombustion engine. The turbocharger does so by pressurizing the intakeair, thereby increasing the mass of air provided to each of the engine'scombustion chambers during the intake stroke. The increased air masssupports combustion of a correspondingly greater amount of fueldelivered to each combustion chamber, which provides increased powerrelative to a naturally aspirated engine of similar displacement. In amotor vehicle, a turbocharged engine may provide increased fuel economyby maintaining a higher power-to-weight ratio than a naturally aspiratedengine of similar output and recovering internal energy from the exhaustto drive the turbocharger compressor. A turbocharger may beadvantageously matched to an engine by creating an ‘operating net,’defining appropriate pressure and flow conditions for the turbochargercompressor between surge and choked flow lines. Proper sizing of theoperating net is required to meet the attributes of the engine: makingthe operating net too large may result in poor response and emissionscharacteristics, for example.

However, a turbocharger compressor coupled to an internal combustionengine may be subject to unwanted surge when a pressure ratio in theturbocharger compressor (viz., P, the ratio of the outlet pressure tothe inlet pressure) is too great relative to the flow of air through theturbocharger compressor. Turbocharger compressor surge (TCS) is adynamic instability mode that can generate air-flow and pressureoscillations of great amplitude; this condition may induce undesirablestresses in the turbocharger and the intake, including excessivetorsional loading on the turbocharger shaft. Continued or excessive TCSmay decrease the longevity of the turbocharger and/or the engine towhich it is coupled. Further, TCS in a motor vehicle may adverselyaffect motorist satisfaction by causing undesirable vibration, noise andpower loss. Turbocharged engine systems may therefore be configured todetect certain kinds of TCS and to take action to suppress TCS when itis detected.

For example, a turbocharged engine system may be configured to sense apressure ratio P and a mass flow rate M of air into the engine intake,and to indicate TCS if the value of P lies outside an intervaldetermined for the value of M. The converse is also possible—indicatingTCS if the value of M lies outside an interval determined for the valueof P. However, such approaches may not be the most suitable for alltypes of TCS.

The pressure-ratio and mass-flow rate intervals referenced above may bedetermined based on steady-state conditions of the engine, where enginespeed and load are related predictably to each other. Under suchconditions, calculations may be used to predict, for any value of M, theappropriate P interval below which TCS will not occur. But TCS may alsooccur during transient states of the engine, where speed and load arenot related to pressure ratio and mass-flow rate as they are understeady-state conditions. Such transient states include, for example,tip-out, rich combustion (intake throttle closed relative to steadystate), and exhaust-gas recirculation (intake throttle open relative tosteady state). Under these conditions and others, methods of TCSdetection based on steady-state P or M intervals may not appropriatelydetect TCS.

To address this issue, some engine systems use P or M intervals derivedfrom steady-state calculations, but build in wide safety margins (e.g.,20%) to guard against transient TCS. This approach, however, maysignificantly limit turbocharger performance and may undermine theadvantages of the turbocharged engine system.

The inventors herein have recognized the inadequacies of the existingmethods outlined above and have provided various approaches directed totransient TCS response. In one embodiment, a method for responding to anexisting or incipient surge condition of a turbocharger coupled to anengine of a motor vehicle is provided. The method comprises receiving asignal responsive to an operating condition of the turbocharger andadjusting one or more operating parameters of the motor vehicle when apower of the signal, integrated over a pre-selected range of non-zerofrequencies, exceeds a pre-selected threshold. Other embodiments providerelated systems for responding to an existing or incipient surgecondition of a turbocharger. The systems and methods disclosed hereinprovide a reliable response to transient TCS whilst avoiding excessiveconstraints on turbocharger performance.

It will be understood that the summary above is provided to introduce insimplified form a selection of concepts that are further described inthe detailed description, which follows. It is not meant to identify keyor essential features of the claimed subject matter, the scope of whichis defined by the claims that follow the detailed description. Further,the claimed subject matter is not limited to implementations that solveany disadvantages noted above or in any part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows in schematic detail an example turbocharged engine system,in accordance with the present disclosure.

FIG. 2 shows a graph of pressure ratio versus corrected mass flow rateof air for various operating states of a hypothetical turbochargedengine, in accordance with the present disclosure.

FIG. 3 shows an example transient TCS observer in schematic detail, inaccordance with the present disclosure.

FIG. 4 shows graphs of engine intake pressure and TCS flag logic versustime for a hypothetical turbocharged engine system, in accordance withthe present disclosure.

FIG. 5 illustrates an example method for indicating transient TCS in aturbocharged engine system, in accordance with the present disclosure.

DETAILED DESCRIPTION

FIG. 1 shows in schematic detail an example engine system 2 includingand engine 3, which may be disposed in a motor vehicle. The enginesystem includes turbocharger compressor 4, which is configured to drawair from air cleaner 6 and to provide pressurized air to intake manifold8. The turbocharger compressor is mechanically coupled to and driven byturbocharger turbine 10 via shaft 12. The turbocharger turbine derivesmechanical power from hot engine exhaust conducted therethrough.Accordingly, turbocharger turbine 10 is configured to admit engineexhaust from exahaust manifold 14, and to route the engine exhaust (at alower temperature and pressure) to exhaust passage 16.

Engine system 2 further includes exhaust-gas recirculation (EGR) valve18, which is an electronically controllable valve configured tocontrollably admit engine exhaust from the exhaust passage toturbocharger compressor 4. Thus, the illustrated engine system embodiesa so-called ‘low-pressure EGR’ strategy, wherein some exhaust may berecirculated from the exhaust (downstream of the turbocharger turbine)to the intake upstream of the turbocharger compressor. Other enginesystems fully consistent with this disclosure may embody a so-called‘high-pressure EGR’ strategy, wherein engine exhaust from an exhaustmanifold (upstream of the turbocharger turbine) may be controllablyadmitted to an intake manifold downstream of a turbocharger compressor,also via a controllable EGR valve.

In the illustrated configuration, at least some of the pressurized airfrom the turbocharger compressor passes through mass flow sensor 20 enroute to the engine. The mass flow sensor may be configured to providean output (e.g., a voltage or current) responsive to the mass flow rateM of air from the turbocharger compressor to the intake of the engine.In some embodiments, the output of the mass-flow sensor may correspondto a mass-flow rate parameter, e.g., a normalized mass flow rate.

FIG. 1 also shows pressure sensor 22, which is coupled to intakemanifold 8 and configured to provide an output (e.g., a voltage orcurrent) responsive to the pressure ratio P. In the illustratedconfiguration, the output of mass flow sensor 20 and the output ofpressure sensor 22 are both routed to electronic control unit 24.Electronic control unit 24 may be any electronic control unit of enginesystem 2 or of the motor vehicle in which the engine system is disposed.In some embodiments, the electronic control unit may be part of a moreextensive electronic system, e.g., a diagnostic system of the motorvehicle. The electronic control unit may be configured to indicate whenturbocharger compressor 4 is undergoing TCS based on the various sensoroutputs provided thereto. The electronic control unit may be furtherconfigured to adjust one or more operating parameters of the motorvehicle to suppress TCS. In some embodiments, such operating parametersmay include engine operating parameters: exhaust gas recirculation,fuel-injection amount, and throttle position, for example. In otherembodiments, the operating parameters may include turbocharger operatingparameters. Therefore, one or more control signals from the electroniccontrol unit may be provided to engage or disengage the turbochargercompressor, to modulate the turbocharger boost and/or speed, to open orclose a turbocharger waste gate or blow-off valve, to open or close anEGR valve, for example. Thus, electronic control unit 24 may beconfigured to indicate when the turbocharger is undergoing TCS and toprovide a surge-suppressing control signal when TCS is indicated.

Further, the electronic control unit may comprise one or more subsystems(e.g., observers) configured to respond to particular variants of TCS.In FIG. 1, for example, electronic control unit 24 includes steady-stateTCS observer 26. The steady-state TCS observer may be coupled to anoutput of mass flow sensor 20 and an output of pressure sensor 22. Thesteady-state TCS observer may be further configured to indicate whetherthe turbocharger compressor at the operating point (M, P) is susceptibleto steady-state TCS, as further described below with the aid of FIG. 2.To this end, the steady-state TCS observer may comprise a set ofelectronic components configured to indicate the surge condition when anintake pressure of the engine exceeds a critical pressure based on amass flow rate of air into the engine, or, when a mass flow rate of airinto the engine is less than a critical mass flow rate based on anintake pressure of the engine, wherein one or more of the criticalpressure and the critical mass flow rate are based on steady-state surgeconditions of the turbocharger compressor.

FIG. 2 shows a graph of pressure ratio P versus corrected mass flow rateM for various operating states of a hypothetical turbocharged engine.The graph includes steady-state TCS boundary 28, which is a line belowwhich TCS cannot occur under steady-state conditions. Above the line,i.e., at higher P or lower M, the turbocharger compressor may besusceptible to TCS. The steady-state TCS boundary may be theoreticallyderived, based on various properties of the turbocharger and the engine,or it may empirically derived, e.g., by operating the turbocharger andengine under steady state conditions, and establishing thresholds wheresteady-state TCS is likely to occur.

For any turbocharged engine system, steady-state TCS observer 26 may beconfigured to determine whether an observed operating point (M, P) ofturbocharger compressor 4 lies above steady-state boundary 28. In doingso, the steady-state TCS observer may employ appropriate digital and/oranalog electronics—digital to analog convertors, logic gates,microprocessors, look-up tables, operational amplifiers, analogmathematical processors, etc. In effect, the steady-state TCS observermay define an interval of allowable P for any observed value of M, andmay indicate TCS when the observed P lies outside that interval. Theconverse is also contemplated; that is, the steady-state TCS observermay define an interval of allowable M for any observed P, and indicateTCS when the observed M lies outside that interval.

FIG. 2 also shows a locus of transient-state operating points 30, whichmay correspond to any series of transient operating states of theturbocharged engine. In the illustrated example, the locus oftransient-state operating points corresponds to a tip-out condition.Even though the entire locus of transient state operating points liesbelow steady-state boundary 28, turbocharger compressor 4 exhibits TCSat some of the operating points. Specifically, transient-state TCS isindicated for points in subset 32 of the locus of transient-stateoperating points.

Transient TCS may be triggered by a sudden and substantial reduction inengine fueling rate and/or exhaust air flow rate. Under typicaloperating conditions, the intake pressure may decay much more slowlyfrom an initial high value than the rate of reduction in turbochargerdriving force. Transient TCS may occur when the accumulated pressure atthe intake exceeds the compressor's ability to sustain positive airmovement.

In order to define appropriate P or M intervals to detect and suppresssuch transient-state TCS in the manner described above, it would benecessary to use a boundary line lower than steady-state boundary 28.For example, transient-safe boundary 34 could be used to defineappropriate P or M intervals below which even transient-state TCS isunlikely to occur. In FIG. 2, transient-safe boundary 34 is derived fromsteady-state boundary 28 by decreasing each pressure value by 20percent.

It is apparent from the graph, however, that the transient-safe boundaryapproach as described above may significantly limit the operating rangeof the turbocharger compressor. Specifically, FIG. 2 shows wasted region36, which lies between steady-state boundary 28 and transient-safeboundary 34. By using the transient-safe boundary to set allowed P or Mintervals for the turbocharger compressor, the turbocharger compressormay not be permitted to operate in the wasted region, even understeady-state conditions. Thus, the performance advantage of theturbocharged engine is significantly reduced relative to its fullpotential.

Therefore, returning now to FIG. 1, electronic control unit 24 alsoincludes transient TCS observer 38. In the illustrated configuration,steady-state TCS observer 26 and transient TCS observer 38 areindependently configured to indicate TCS. In other embodiments, however,two or more TCS observers may be configured for various modes ofinteroperability. Through appropriate logic circuitry 40, an indicatingsignal from either observer may trigger adjustment of one or moreoperating parameters of the motor vehicle. For example, the indicatingsignal may invoke a TCS flag 42 in the electronic control unit or in adiagnostic system of the motor vehicle. The indicating signal mayfurther provide a surge-suppressing control signal to the turbocharger.

The surge-suppressing control signal may suppress TCS in a number ofdifferent ways, depending on the particular configuration of theturbocharged engine system. In some embodiments, the surge-suppressingcontrol signal may trigger a reduction in turbocharger boost and/orspeed. In the illustrated embodiment, engine system 2 includes wastegate 43, configured to controllably cause some of the engine exhaust toby-pass turbocharger turbine 10, thereby providing less torque to theturbocharger compressor. The engine system also includes blow-off valve44, configured to vent some of the compressed air from the turbochargercompressor away from the intake manifold—e.g., to atmosphere or back tothe turbocharger compressor inlet. Thus, the surge-suppressing controlsignal may have the effect, via the controller 24, of opening the wastegate and/or the blow-off valve to reduce the turbocharger boost and/orspeed.

In other embodiments, the surge-suppressing control signal may have theeffect, via the controller 24, of adjusting exhaust-gas recirculation byat least partly opening or closing low pressure EGR valve 18.Alternatively or additionally, a high pressure EGR valve may be adjustedbased on the surge-suppressing control signal.

More specifically, some operating conditions of the engine system may besuch that TCS is suppressed by further opening the EGR valve, and other(different) operating conditions may be such that TCS is suppressed byfurther closing the EGR valve. Therefore, engine system 2 includes EGRflow sensor 46, which is coupled to exhaust passage 16. The EGR flowsensor may be any component responsive to the direction of EGR flow intoor out of the exhaust passage. In the illustrated embodiment, an outputof the EGR flow sensor is routed to EGR control module 48 of electroniccontrol unit 24. The EGR control module may be configured to at leastpartly close EGR valve 18 in response to the surge-suppressing controlsignal when EGR flow sensor 46 indicates positive exhaust-gasrecirculation flow. Further, the EGR control module may be configured toat least partly open EGR valve 18 in response to the surge-suppressingcontrol signal when EGR flow sensor 46 indicates negative exhaust-gasrecirculation flow.

FIG. 3 shows an example transient TCS observer 38 in schematic detail.The transient TCS observer includes first and second frequency-selectivemodules, a power-responsive module, and a discriminating module. In theillustrated embodiment, the first frequency-selective module comprisesband-pass filter 49. The band-pass filter is configured to receive anoutput from pressure sensor 22. The band-pass filter may be an analogfilter having a fixed or adjustable lower pass-band edge frequency and afixed or adjustable upper pass-band edge frequency. In one embodiment,the lower pass-band edge frequency of the band-pass filter may be 30radians per second, and the upper pass-band edge frequency may be 80radians per second. In particular, the band-pass filter may be asecond-order Butterworth filter, a Chebyshev filter, or an ellipticfilter, for example. While FIG. 3 shows band-pass filter 49 configuredto receive an output from pressure sensor 22, the band-pass filter orother first frequency-selective module may instead be configured toreceive an output from mass air flow sensor 20, for example, orotherwise configured to respond to one or more of an intake pressure ofthe engine and a mass flow rate of air into the engine.

Continuing in FIG. 3, the first frequency-selective module is configuredto provide an output to a power-responsive module. The power-responsivemodule may be any module responsive to the power of the sensor outputwithin a band of frequencies selected via the first frequency-selectivemodule. In the illustrated embodiment, the power-responsive modulecomprises absolute-value convertor 50. The absolute-value convertor maybe an analog circuit configured to accept a positive or negative input(e.g., voltage input) from the first frequency-selective module and togenerate a corresponding positive output.

Continuing in FIG. 3, the power-responsive module is configured toprovide an output to a second frequency-selective module. In theillustrated embodiment, the second frequency-selective module compriseslow-pass filter 51. The low-pass filter may be an analog filter having afixed or adjustable lower pass-band edge frequency. In one embodiment,the pass-band edge frequency of the low-pass filter may be 30 radiansper second. In one particular embodiment, the band-pass filter may be asecond-order Butterworth filter.

Continuing in FIG. 3, the second frequency-selective module isconfigured to provide an output to discriminating module 52. In theillustrated embodiment, the discriminating module comprises comparator53, reference 54, adder 55, and increment 56. The comparator may beconfigured to compare the output of the second frequency-selectivemodule to reference 54, which may be a fixed or adjustable voltagereference.

In one embodiment, the comparator may be configured to change an outputvoltage state (negative to positive, for example) when the output of thesecond frequency-selective module exceeds the reference. The comparatoris further configured to provide an output to adder 55. The adder isconfigured to add the output of the comparator to an increment 56, whichmay be fixed or adjustable voltage increment, for example. In oneembodiment, the increment may be of approximately the same magnitude asthe output voltage swing of comparator 53. Thus, the discriminatingmodule may be configured to indicate the surge condition when an outputof the second frequency-selective module exceeds a threshold. Thediscriminating module may be further configured to provide asurge-suppressing control signal to the turbocharger when the surgecondition is indicated.

While the above embodiment illustrates the use of analog electroniccircuitry in the various modules of transient TCS observer 38, it willbe understood that an equivalent functionality may be enacted viadigital electronics and digital algorithms. For example, one or moresensor signals may be subject to analog-to-digital conversion andprocessed via an on-board computer in the electronic control unit. Theon-board computer may be configured to enact appropriatefrequency-selective and power-responsive algorithms to provide theindicated functionality.

FIG. 4 shows graphs of intake pressure and TCS flag logic versus timefor a hypothetical turbocharged engine system. First plot 58 of FIG. 4is a plot of boost pressure versus time over a period that includes atransient TCS event 60. Second plot 62 is a plot of the logic state ofTCS flag 42, through which electronic control unit 24 indicates TCS. Inthe illustrated example, the transient TCS observer is triggeredapproximately 100 milliseconds into TCS event 60, and indicates theevent by changing the state of TCS flag 42.

FIG. 5 illustrates an example method 64 for responding to transient TCSin a turbocharged engine system. While method 64 is discussed presentlywith reference to the example configurations presented hereinabove, itwill be understood that the method may be enacted by various otherconfigurations as well.

Method 64 begins at 65, where an output signal of a sensor is receivedin an electronic control unit of a motor vehicle. In some embodiments,the output signal may be responsive to a pressure downstream of theturbocharger compressor, downstream of a charge air cooler (not shown inthe drawings), and/or upstream of the intake manifold, for example. Inanother embodiment, the output signal may be responsive to a mass flowrate of air into the engine intake, or to a normalized mass-flowparameter. The method advances to 66, where band-pass filtering isapplied to the signal. Band pass filtering may be applied via analogcircuitry, as described above, or it may be applied digitally, viaanalog-to-digital conversion followed by Fourier filtering, for example.

Method 64 continues to 68, where the absolute value of theband-pass-filtered signal is computed. The method continues to 70, wherelow-pass filtering is applied to the absolute value determined at 68.Steps 68 and 70 may be enacted via appropriate analog and/or digitalelectronics, as described hereinabove. Taken together, steps 66-70 ofthe method may yield an estimate of the power of the signal receivedintegrated over a pre-selected range of non-zero frequencies. Further,the preselected range of non-zero frequencies may be defined by the passband applied in step 66. In other embodiments, different estimates ofthe power of the signal integrated over the pre-selected range ofnon-zero frequencies may be computed. For example, a different estimateof the power may be computed according to a modified method in which thesquare of the band-pass filtered signal is used instead of the absolutevalue.

Method 64 then continues to 72, where it is determined whether thelow-pass filtered result computed at 70 exceeds a threshold. If thelow-pass filtered result exceeds the threshold, then at 74, a TCS flagin a diagnostic system of the motor vehicle is set. The method thenadvances to 76, where a blow-off valve in the turbocharger is opened,thereby venting some of the excess pressure that would normally beprovided to the intake of the engine. This is one way in which the surgecondition may be suppressed. In other embodiments, one or more otheroperating parameters of the motor vehicle may be adjusted when a powerof the signal in the pre-selected range of non-zero frequencies exceedsa pre-selected threshold. Adjusting such other operating parameters maycomprise, for example, opening a waste gate in the turbocharger,decreasing a boost and/or speed of the turbocharger, momentarily openingor closing an EGR valve, or taking any other action to suppress thesurge condition. In one embodiment, adjusting the one or more operatingparameters may comprise at least partly closing an exhaust-gasrecirculation valve when positive exhaust-gas recirculation flow isindicated, and, at least partly opening an exhaust-gas recirculationvalve when negative exhaust-gas recirculation flow is indicated.Following this action, or if it is determined that the low-pass filteredresult does not exceed the threshold, then the method returns.

It will be understood that method 64 or related methods may be used inconjunction with other methods applicable to steady-state TCS detection.Thus, various contemplated methods may further comprise adjusting theone or more operating parameters when an intake pressure of the engineexceeds a critical pressure based on a mass flow rate of air into theengine, or, when a mass flow rate of air into the engine is less than acritical mass flow rate based on an intake pressure of the engine,wherein one or more of the critical pressure and the critical mass flowrate are based on steady-state surge conditions of the turbochargercompressor.

It will be understood that the example control and estimation routinesdisclosed herein may be used with various system configurations. Theseroutines may represent one or more different processing strategies suchas event-driven, interrupt-driven, multi-tasking, multi-threading, andthe like. As such, the disclosed process actions may represent code tobe programmed into computer readable storage medium in an electroniccontrol unit. It will be understood that some of the process stepsdescribed and/or illustrated herein may in some embodiments be omittedwithout departing from the scope of this disclosure. Likewise, theindicated sequence of the process steps may not always be required toachieve the intended results, but is provided for ease of illustrationand description. One or more of the illustrated actions, functions, oroperations may be performed repeatedly, depending on the particularstrategy being used.

Finally, it will be understood that the systems and methods describedherein are exemplary in nature, and that these specific embodiments orexamples are not to be considered in a limiting sense, because numerousvariations are contemplated. Accordingly, the present disclosureincludes all novel and non-obvious combinations and sub-combinations ofthe various systems and methods disclosed herein, as well as any and allequivalents thereof.

1-8. (canceled)
 9. A system for responding to an existing or incipientsurge condition of a turbocharger, the turbocharger coupled to an engineof a motor vehicle, the system comprising: a first frequency-selectivemodule coupled to an output of a sensor, the sensor responsive to one ormore of an intake pressure of the engine and a mass flow rate of airinto the engine; a power-responsive module coupled to an output of thefirst frequency-selective module; a second frequency-selective modulecoupled to an output of the power-responsive module; and adiscriminating module configured to indicate the surge condition when anoutput of the second frequency-selective module exceeds a threshold andto adjust an operating condition responsive to the indicated surgecondition.
 10. The system of claim 9, wherein the firstfrequency-selective module comprises a band-pass filter, thepower-responsive module comprises an absolute-value convertor, and thesecond frequency-selective module comprises a low-pass filter.
 11. Thesystem of claim 10, wherein the band-pass filter is a second-orderButterworth filter.
 12. The system of claim 10, wherein a pass band ofthe band-pass filter is substantially between 30 and 80 radians persecond.
 13. The system of claim 9, wherein the discriminating module isfurther configured to provide a surge-suppressing control signal to oneor more of the turbocharger and the engine when the surge condition isindicated.
 14. The system of claim 9, further comprising athreshold-testing module configured to indicate the surge condition whenan intake pressure of the engine exceeds a critical pressure based on amass flow rate of air into the engine, or, when a mass flow rate of airinto the engine is less than a critical mass flow rate based on anintake pressure of the engine, wherein one or more of the criticalpressure and the critical mass flow rate are based on steady-state surgeconditions of the turbocharger.
 15. A system comprising: an engine; aturbocharger coupled to the engine; a sensor configured to furnish anoutput responsive to one or more of an intake pressure of the engine anda mass flow rate of air into the engine; a observer comprising aband-pass filter configured to receive the output, an absolute-valueconvertor configured to receive an output of the band-pass filter, alow-pass filter configured to receive an output of the absolute-valueconvertor, and a first set of electronic components configured toindicate the surge condition and to adjust an operating condition whenan output of the low-pass filter exceeds a threshold.
 16. The system ofclaim 15, further comprising a diagnostic system, wherein the first setof electronic components is further configured to set a flag in thediagnostic system when the surge condition is indicated.
 17. The systemof claim 15, further comprising a waste gate, wherein the first set ofelectronic components is further configured to at least partly open thewaste gate when the surge condition is indicated.
 18. The system ofclaim 15, further comprising an exhaust-gas recirculation valveconfigured to controllably conduct at least some engine exhaust to anintake of the engine, wherein the first set of electronic components isfurther configured to at least partly close the exhaust-gasrecirculation valve when the output of the low-pass filter exceeds athreshold during positive exhaust-gas recirculation flow, and, to atleast partly open the exhaust-gas recirculation valve when the output ofthe low-pass filter exceeds a threshold during negative exhaust-gasrecirculation flow.
 19. The system of claim 15, further comprising ablow-off valve, wherein the first set of electronic components isfurther configured to at least partly open the blow-off valve when thesurge condition is indicated.
 20. The system of claim 16, the observerfurther comprising a second set of electronic components configured toindicate the surge condition when an intake pressure of the engineexceeds a critical pressure based on a mass flow rate of air into theengine, or, when a mass flow rate of air into the engine is less than acritical mass flow rate based on an intake pressure of the engine,wherein one or more of the critical pressure and the critical mass flowrate are based on steady-state surge conditions of the turbocharger. 21.A system comprising: an engine with a turbocharger having a turbine anda compressor; and a controller having computer readable medium includinginstructions for responding to turbocharger surge, includingrecirculating exhaust from downstream of the turbine to upstream of thecompressor; and opening a blow-off valve at the compressor when a powerof a turbocharger condition integrated over a pre-selected range ofnon-zero frequencies exceeds a pre-selected threshold.